Conventionally, a stacked gate electrode structures composed of a floating gate electrode layer and a control gate electrode layer has been employed to render non-volatile storage of information. In realizing the stacked gate electrode structure, an inter-gate insulating film is formed between the floating gate electrode layer and the control gate electrode layer to provide insulation between the layers.
As a result of recent attempts in exploring gate insulating films exhibiting higher performance, ONO film (stack of silicon oxide film, silicon nitride film, and silicon oxide film) has been generally employed as the inter-gate insulating film. However, increasing integration of gate electrodes has given rise to occurrence of bird's beak in the ONO film and difficulties in achieving higher densification due to failure in obtaining the desired coupling ratio. As disclosed in JP H09-219459 A, it has beer considered to employ NONON stacked film structure (silicon nitride film/silicon oxide film/silicon nitride film/silicon oxide film/silicon nitride film) for the inter-gate insulating film. According to JP H09-219459 A, TDDB (Time Dependant Dielectric Break Down) can be reduced by a magnitude of approximately one digit as compared to conventional ONO film, thereby preventing the occurrence of bird's beak.
However, it has been observed that due to some unknown reason, employing NONON stacked film structure for inter-gate insulating film increases the occurrence of charge transportation between the neighboring floating gate electrodes when the state of charge accumulation between the neighboring floating gate electrode layers differ. Thus, when the charge accumulation between the floating electrode layers differ, charge transportation may undesirably lead to problems such as change in threshold voltage and data error.